i asked this once to some guy selling a set for 800 dollars.
and the answer he gave which seems 99% plausible is
"demand"
it has the supra name to it, everyone wants them and not many people get rid of them.
same scenario goes for supra front cut...8 grand for a 92 model supra...compare to soarer ~3 grand (a front cut that is not as in demand but contains still good engine nonetheless...yes i know the engine isn't quite the same in the 2 models but it is the best comparison i can come up with) or an sr20-det s13 half cut (commonly more than a soarer half cut).
with simple answer being demand.
people know they can get the money because they know people pay it.
my 2 cents
as for who makes them....ummm. no idea...i think sumitomo make a lot of OEM callipers though (including r33 gts calliper)